Electromagnetic components such as transformers have traditionally been constructed by winding one or more conductors about a cylindrical or toroidal core. This method of construction requires that a conductor, such as a wire, be wrapped around the outer surface of the core. The resulting components are expensive and time consuming to manufacture, and do not readily lend themselves to miniaturization or automated assembly.
More recently, electromagnetic components have been constructed using printed circuit board (PCB) manufacturing techniques, where windings and individual winding turns are formed from a stack of PCB layers wherein each layer includes one or more conducting traces patterned on the surface of the PCB layer, or formed from a multi-layer PCB having such conducting traces on each layer. The use of PCB conductive traces as windings has several advantages over conventional, wound windings. First, the assembled PCB winding has a smaller mounting footprint than conventional windings, since it does not need extra leads or soldering pads. Second, the PCB winding assembly is much simpler than conventional windings, since the winding and other components in the winding circuit of a multilayer PCB can be board mounted using the same reflow and automation processes used to mount other components. Third, a multi-layer PCB winding has improved reliability since the likelihood of shorting across adjacent turns of the winding is greatly reduced or substantially eliminated.
In a multi-layer PCB, a PCB winding is formed from a plurality of patterned conductive traces, typically of copper, each formed on a separate insulating layer of the multi-layer PCB. Each trace forms a nearly closed typically circular pattern, so as to create the electromagnetic equivalent of one turn or loop of a prior art wire formed winding. Terminal points are formed at the ends of each trace for making connections to other traces on other layers, so as to form the individual turns of the winding. For example, the pattern can be a “C” shape with a terminal point at each of the two extreme points of the C. The PCB winding is formed by connecting the traces from different layers of the PCB through the intervening insulating PCB layers. These connections are typically plated through holes or vias in the PCB insulating layers. The traces can be connected in various ways. The traces can all be connected in series to form a winding where each trace is a separate turn of the winding. In this example, the terminal ends of each trace are offset from the traces on the adjacent levels, so that the plated through holes in each level do not intersect. Two or more traces can also be connected in parallel to decrease the impedance of a particular turn of the winding. In yet another alternate embodiment, one or more of the traces can be formed as separate windings. In each case, the resultant winding (or windings) is a function of the way in which the conductive traces on each layer of the multi-layer PCB are connected together and coupled to external circuits, to thereby create a planar transformer.
The inductance of a winding formed using a multi-layer PCB can be increased by introducing a core of a magnetic material through an aperture formed in the PCB layers that extends through a central non-conducting region of each layer. Alternatively, the core can be configured to surround the PCB. The core is typically included as part of a housing for the multi-layer PCB winding. Conductive leads or vias are included on one or more of the PCB layers to enable the efficient electrical connection of the PCB winding to an external circuit, for example, by surface mounting and reflow soldering of the PCB winding to another PCB having other circuit components. This use of a multi-layer PCB to fabricate electromagnetic components results in smaller, more easily manufactured, and more reproducible components than is possible using a winding formed from a wire wrapped about a core.
In order to achieve better coupling and to reduce the leakage inductance of the transformer, the primary and secondary windings of the transformer are typically placed in close proximity to one another. One drawback of this arrangement is that it increases the capacitive coupling between the primary and secondary windings, which results in the generation of increased electromagnetic interference (EMI). That is, due to the inter-winding capacitance of the transformer, common mode noise will be injected into the secondary. In a planar, low profile transformer required for low profile packaging, this inter-winding capacitance is larger and, as a result, the common mode noise injection via this parasitic capacitance is larger.
This drawback is especially significant for a two switch forward converter. Unlike in a single switch forward converter, the primary winding in a two switch forward converter is not connected to either the positive or the return side of the converter's input voltage. The switches in the two switch forward converter are typically MOSFETs. The converter having MOSFET switches is also referred to herein as a two FET forward converter.
FIG. 1 shows a prior art two FET forward converter 10. The converter 10 has an input terminal 14 to which an input DC voltage, Vin, is coupled, relative to a ground potential at an input terminal 16, and an output terminal 32 where the output DC voltage, VOUT, is provided relative to ground. Converter 10 includes a transformer 42 having primary winding 2 and a secondary winding 6. Each winding has a first and second end. A first power switch 34 is coupled between the first end of primary winding 2 and input terminal 14. A second power switch 36 is connected between the second end of primary winding 2 and input terminal 16. Power switch 34 is connected in series with primary winding 2 and power switch 36 across the input DC voltage terminals. A diode 18 is connected between the second end of primary winding 2 and input terminal 14. The diode 22 is connected between the first end of primary winding 2 and input terminal 16. Each of the power switches 34, 36 is preferably a MOSFET having a source, a drain, and a gate. A controller (not shown) preferably provides a control signal, e.g. a pulse width modulated (PWM) signal, coupled to each control input of power switches 34 and 36.
On the secondary side of the forward converter 10, transformer 42 has a secondary winding 6 having a second end connected to output terminal, 38. Converter 10 includes an inductor 24 connected in series with a diode 26 between output terminal 32 and the first end of secondary winding 6. A capacitor 28 is connected across the output terminals 32, 38. A diode 44 is connected between the junction of the cathode of diode 26 and inductor 24 and output terminal 38.
As shown in FIG. 1, converter 10 has a primary winding 2 having two terminals 7 and 9. Primary winding terminal 7 is connected to the source terminal of switch 34. Primary winding terminal 9 is connected to the drain terminal of switch 36. For the two switch forward converter 10, the voltage swing at primary winding terminals 7 and 9 is at a maximum during normal operation. If primary winding terminals 7 and 9 are located near the secondary winding 6 of transformer 42, a significant amount of common mode noise is coupled from the primary side to the secondary side of the transformer 42 due to the capacitance between primary winding 2 and secondary winding 6. This coupled common mode noise increases EMI for converter 10.
U.S. Pat. No. 5,990,776 (“the '776 patent”) discloses a single ended switch forward converter that includes one FET switch for switching the primary winding. The '776 patent discloses a primary-secondary-primary (“pri-sec-pri”) type transformer construction. The '776 patent discloses a transformer wherein all of the primary and secondary windings are integrated in a PCB.
The '776 patent teaches that the top winding 72 connected to the input voltage source is the quiet area of the primary winding since it exhibits a lower voltage swing, and that therefore it is logical to locate the secondary in the vicinity of winding 72. However, due to reasons of symmetry, the secondary winding 80 in '776 is positioned between primary windings 74 and 76.
Unlike in the single switch forward converter for which the '776 patent teachings were directed, the primary winding in a two switch forward converter is not connected to either the positive or the return side of the converter's input voltage. One drawback of the '776 patent, therefore, is that it does not address the unique problems in reducing common mode noise for a two switch forward converter. The '776 patent does not disclose, for instance, the optimum location for the secondary winding in a two switch forward converter.
U.S. Pat. No. 6,211,767 discloses a transformer having a secondary copper strip mounted and fixed on the primary winding PCB by means of solderable via holes, but does not disclose a design to significantly reduce common node noise.
A need therefore exists to reduce common mode noise for a planar transformer. The need especially exists to reduce common mode noise for a planar transformer designed for use in two FET forward converters and which can also be used in single ended, half bridge converters and push pull converters.